1. Field of the Invention
This invention relates to screw-thread protection.
2. Summary of the Prior Art
It is conventional practice in tubing (pipeline) couplings to make use of a dope or sealant applied to mating screw threads with the intention of lubricating the threads and minimizing the risk of leakage. Such dopes may give rise to no difficulty in use when the internal pressure in the pipeline is low and the ambient temperature is modest (i.e. room temperature) but substantially greater difficulty may be encountered where high pressures and temperatures are involved such as, for example, in production and reinjection tubing strings for oil and gas wells.
The problems of high pressures in gas and oil wells have resulted in the development of lubricants (or dopes) with a hydrocarbon grease base and with the addition of comparatively large particles (of the order of 100 microns) of copper, graphite, zinc, lead or other materials or mixtures thereof. These will tend to accumulate in the root helices of the mating screw threads and in theory these particles will prevent the establishment of helical leakage paths. However, the effect of high temperatures results in a lowering of the viscosity of the grease base and in conjunction with the high pressure allows a leakage path to be established between the comparatively large particles. The leakage path along the screw-thread would have no serious consequence provided that the especially provided sealing surfaces were themselves fully engaged and thus effective, but there is a possible risk that the sealing surfaces themselves are not, in practice, fully effective if the tightening torque has not fully engaged the thread sealing surfaces or other final abutment of the two mating threads. There is also a risk that the sealing faces themselves may be damaged.
Tests have been applied to tubing strings (pipelines) prior to installation to detect any existing leaks, but the dopes hitherto used have masked the leaks which only become apparent when the tubing is in use. Thus, conventional and previously proposed dopes which ostensibly prevent leakage in damaged or defective couplings, in practice merely mask the leaks until the tubing is run in the well at which time it is difficult and expensive to remedy the leaks.
It would therefore be desirable, when testing tubing connections for leak tightness, to avoid the use of dopes with coarse particles and a viscous carrier which can temporarily prevent a leakage flow passing through.
The use of liquid lubricants effective to prevent direct physical contact between relatively rotating parts is so conventional in engineering practice over hundreds, if not thousands of years that detailed discussion is superfluous. Over the past thirty years it has become common practice to add to liquid lubricants, what are known as dry film lubricants which form deposits on surfaces to be protected against galling in the event temporary overload conditions. In all rotary bearing situations however, it has been the standard practice to add the dry film lubricant to the liquid lubricant. There would be no gain, and some additional cost of the mating parts were to be coated with dry film lubricant in a separate process. It follows that the previously very well known practices in the rotary bearing art provide little or no guidance to the person of skill who is faced with the problem of overcoming the defects of "dopes", well known in the art of preventing leaks in tubing or piping screw-threaded points.
An object of the present invention is to provide a method of protecting screw-threads and of reducing the risk of masking eventual leaks along the mated threads.